The present disclosure relates to the subject matter contained in Japanese Patent Application No.2001-211314 filed on Jul. 11, which is incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to a radar apparatus using frequency modulation (FM) such as an FM-CW system and in particular to detection and correction of the frequency modulation characteristic thereof.
2. Description of the Related Art
Hitherto, an FM-CW system radar 1 having a basic configuration as shown in FIG. 16 has been mainly installed in an automobile for use to give a collision alarm, prevent or lighten a collision, perform vehicle-to-vehicle control of auto cruise control, drive a car, etc. FIG. 16 (a) shows a schematic electric configuration and FIG. 16 (b) shows a modulation signal waveform. Related arts to the FM-CW system radar are disclosed in JP-A-5-40169, JP-A-7-55942, JP-A-8-327728, etc., for example. JP-A-5-40169 discloses an art for using second frequency modulation to improve the reception S/N ratio.
In the basic configuration of the FM-CW system radar 1, a radio wave is transmitted from a transmission antenna 2 and the reflected radio wave from a target, etc., is received at a reception antenna 3, as shown in FIG. 16 (a). A high-frequency signal of a millimeter waveband generated from a VCO (voltage-controlled oscillator) 4 is given to the transmission antenna 2. A part of the high-frequency signal for exciting the transmission antenna 2 from the VCO 4 branches from a coupler 5 and is mixed with a reception signal from the reception antenna 3 by a mixer 6. An output signal from the mixer 6 is selected through a BPF (band-pass filter) 7 and is amplified by an amplifier 8. The high-frequency signal generated from the VCO 4 is subjected to frequency modulation in accordance with the voltage level of a modulation signal given by a modulation signal generation circuit 9.
For example, in the FM-CW system radar 1 of a millimeter waveband, a modulation signal shaped like a triangular wave of about several hundred Hz as shown in FIG. 16 (b) is used to generate an FM wave with the maximum frequency shift amount being several ten to several hundred MHz. As the modulation signal, a saw tooth wave (chirp wave) maybe used in some cases. If the frequency modulation characteristic of the VCO 4 has good linearity relative to change in the voltage level of the modulation signal, the frequency of the high-frequency signal generated from the VCO 4 also changes linearly corresponding to FIG. 16 (b). The frequency of the reflected radio wave received at the reception antenna 3 is delayed from the frequency of the high-frequency signal given to the transmission antenna 2 as much as the time taken for the radio wave to go and back at the distance to the target. If the high-frequency signal whose frequency changes like a triangular wave corresponding to FIG. 16 (b) is generated from the VCO 4 and the distance to the target is constant, the signal output from the mixer 6 contains a beat signal component of a constant frequency corresponding to the time taken for the radio wave to go and back at the distance. The beat signal component is selected through the BPF 7 and is amplified by the amplifier 8 and then can be input to a signal processing circuit 10 for calculating the distance to the target. When the distance to the target changes, the effect of Doppler shift appears in the frequency of the beat signal and the relative speed can also be calculated by the signal processing circuit 10. Letting the frequency of the beat signal be fb, the frequency depending on the distance be fx, and the frequency depending on the relative speed be fd,
fb=fxxc2x1fd. 
With the FM-CW system radar 1, the frequency modulation characteristic in the VCO 4 has an importance effect on the measurement accuracy. JP-A-7-55942 discloses an art for previously measuring the frequency modulation characteristic of a voltage-controlled oscillator for generating a high-frequency signal of an FM-CW radar and making a correction with an inverse function of the measured characteristic for improving linearity. JP-A-8-327728 discloses an art for correcting a modulation signal so that the frequency of a high-frequency signal generated in an FM-CW radar apparatus changes like a triangular wave. JP-A-6-34756 discloses an art wherein the linearity of a voltage-controlled oscillator for generating a high-frequency signal as a source of a transmission radio wave in a radar transponder for transmitting a frequency-modulated radio wave and responding upon reception of a radio wave from a radar is corrected with data previously stored in memory. However, the related arts do not give any direct description as to how the frequency modulation characteristic is measured.
FIG. 17 shows a schematic configuration for measuring the frequency modulation characteristic and sensing whether or not linearity is maintained in the FM-CW system radar 1 in the related art shown in FIG. 16 (a). Another signal source 11 is provided, the high-frequency signal from the VCO 4 is branched by a coupler 12 and is mixed by a mixer 13 for down conversion, and the difference from the frequency of the signal source 11 is counted by a counter 14. In addition to such down conversion, a method of counting the frequency of the high-frequency signal from the VCO 4 is also available.
The FM-CW system radar 1, etc., installed in a vehicle is used in a hostile environment concerning vibration, temperature, etc. Thus, if the linearity of the frequency modulation characteristic is good at the initial stage, there is a possibility of degradation while the radar is used. If the down converter configuration as shown in FIG. 17 is adopted to install a configuration for detecting the linearity of the frequency modulation characteristic in the FM-CW system radar 1 itself, as the signal source 11, the mixer 13, and the like, expensive components for high frequencies of a millimeter waveband become necessary. To count the frequency of the high-frequency signal from the VCO 4, the millimeter waveband cannot directly be counted and therefore a frequency divider needs to be used. However, the frequency divider operating in the millimeter waveband is expensive and as the frequency dividing ratio increases, the measurement accuracy is degraded.
That is, if an attempt is made to detect and correct the frequency modulation characteristic in the system in the related art as shown in FIG. 17, the following problems are involved:
(1) The higher the transmission frequency, the higher the costs of the components, such as a detector and a frequency divider.
(2) The higher the transmission frequency, the larger the frequency dividing ratio and the worse the measurement accuracy.
It is an object of the invention to provide a radar apparatus that can detect and correct the frequency modulation characteristic in a simple configuration at low costs.
According to the invention, there is provided a radar apparatus for performing frequency modulation of a high-frequency signal, transmitting the frequency-modulated signal, and receiving a reflected radio wave to detect a target, the radar apparatus including a high frequency generation section for generating the high-frequency signal, a modulation signal generation section for generating and giving a modulation signal to the high frequency generation section to modulate the high frequency signal, a mixing section for mixing the high-frequency signal and the reception signal of the reflected radio wave, and a signal processing section for controlling the modulation signal generation section so as to give a modulation signal for detection changing among a plurality of predetermined signal levels and retained for a predetermined time for each signal level to the high frequency generation section, detecting frequency of a differential signal between the high-frequency signal and the reception signal, inputted from the mixing section, and detecting a frequency modulation characteristic of the high frequency generation section based on the relationship between a signal level of the modulation signal for detection and the frequency of the differential signal.
According to the invention, the radar apparatus for performing frequency modulation of a high-frequency signal, transmitting the frequency-modulated signal, and receiving a reflected radio wave for detecting a target comprises the modulation signal generation section, the mixing section, and the signal processing section. The modulation signal generation section generates a modulation signal whose change state can be controlled and gives the signal to the high frequency generation section. The mixing section mixes the high-frequency signal generated from the high frequency generation section and the reception signal of the reflected radio wave. The signal processing section controls the modulation signal generation section so as to give modulation signal for detection changing between a plurality of predetermined signal levels and retained for a predetermined time for each signal level to the high frequency generation section, inputs the signal of the component of the difference between the high-frequency signal and the reception signal from the mixing section, detects the frequency, and detects the frequency modulation characteristic of the high frequency generation section based on the relationship between the signal level of the modulation signal for detection and the frequency. The modulation signal for detection generated from the modulation signal generation section is changed between a plurality of predetermined signal levels and is mixed with the reception signal from the target by the mixing section, the frequency of the signal of the component of the difference is detected, the relationship between the signal level of the modulation signal for detection and the frequency is found, and the frequency modulation characteristic of the high frequency generation section can be detected. The reception signal can be used as the high-frequency signal source to detect the frequency modulation characteristic and the mixing section for reception can also be shared, so that the frequency modulation characteristic can be detected easily in the inexpensive configuration.
The invention provides the radar apparatus further including a modulation characteristic correction section for correcting the modulation signal based on the frequency modulation characteristic detected by the signal processing section so that the frequency modulation characteristic is not placed out of a predetermined normal range.
According to the invention, the modulation signal is corrected so that the frequency modulation characteristic that can be detected easily in the inexpensive configuration is not placed out of the predetermined normal range, so that the frequency modulation characteristic can also be corrected easily in the inexpensive configuration.
The invention provides the radar apparatus further including a width calculation section for calculating frequency modulation width, which is a difference between frequency of the differential signal when the signal level of the modulation signal is the maximum value and frequency of the differential signal when the signal level is the minimum value based on the frequency modulation characteristic detected by the signal processing section.
According to the invention, the frequency modulation width of the difference between the frequency when the signal level of the modulation signal is the maximum value and the frequency when the signal level is the minimum value can be calculated based on the frequency modulation characteristic detected as frequency change between a plurality of signal levels.
In the radar apparatus of the invention, the width calculation section includes a counter for counting the frequency of the difference.
According to the invention, frequency change between a plurality of signal levels is counted by the counter, so that frequency change can be measured easily.
The invention provides the radar apparatus further including a width correction section for correcting the modulation signal based on the frequency modulation width calculated by the width calculation section so that the frequency modulation width has a predetermined value.
According to the invention, the frequency modulation width of the frequency modulation characteristic can also be corrected to the predetermined value. When the distance is calculated in the signal processing section, a correction can also be made on the arithmetic operations.
In the radar apparatus of the invention, one of the modulation characteristic correction section and the width correction section performs the correction when the level of the reception signal of the reflected radio wave is equal to or greater than a predetermined level.
According to the invention, if the level of the reception signal is equal to or greater than the predetermined level, a correction is made and thus can be made stably.
The invention provides the radar apparatus further including further comprising a temperature detection section, in which one of the modulation characteristic correction section and the width correction section performs the correction when a temperature detected by the temperature detection section has a predetermined temperature.
According to the invention, if the frequency of the high-frequency signal generated from the high frequency generation section changes with the temperature, a correction is made if the predetermined temperature of the temperature detection section is reached, so that the effect of the temperature can be decreased and the accuracy of the frequency modulation characteristic can be enhanced.
In the radar apparatus of the invention, one of the modulation characteristic correction section and the width correction section performs the correction when a distance to the target is in a predetermined distance range.
According to the invention, the distance range, etc., having a high possibility that the signal level of the reception signal will lower is placed out of the predetermined distance range, whereby correction processing is not entered, so that accuracy degradation of correction can be avoided.
In the radar apparatus of the invention, one of the modulation characteristic correction section and the width correction section performs the correction when relative speed of the target is in a predetermined speed range.
According to the invention, the relative speed of the target is considered and if the relative speed of the target is placed in the predetermined speed range, a correction is made, so that the accuracy of the correction can be enhanced.
In the radar apparatus of the invention, the signal processing section detects the frequency modulation characteristic when an input level of the reflected radio wave from the target is equal to or greater than a predetermined reference level at a normal radar operation time.
According to the invention, if the input level of the reception signal is equal to or greater than the predetermined reference level, the frequency modulation characteristic is detected, so that the radar apparatus can concentrate attention on usual target detection processing.
In the radar apparatus of the invention, the signal processing section detects the frequency modulation characteristic when a distance to the target is in a predetermined distance range.
According to the invention, if the distance to the target is out of the predetermined distance range, the frequency modulation characteristic is not detected, so that the radar apparatus can concentrate attention on usual target detection processing.
In the radar apparatus of the invention, wherein the signal processing section detects the frequency modulation characteristic just after detecting the target initially after a radar operation starts and determines whether or not a normal modulation operation for the high frequency generation section is performed.
According to the invention, if the target is detected initially and it is made possible to receive the reflected radio wave, whether or not the frequency modulation characteristic of the high frequency modulation section is normal is determined, so that occurrence of an anomaly can be found at an early stage if the anomaly occurs.
The invention provides the radar apparatus further including a reference signal source for generating a reference signal to decrease a frequency with respect to the differential signal between the high-frequency signal and the reception signal, the differential signal inputted to the signal processing section from the mixing section, in which the signal processing section detects the frequency modulation characteristic based on the inputted differential signal with the frequency decreased by the reference signal.
According to the invention, for the component of the difference between the high-frequency signal and the reception signal provided by the mixing section, the reference signal generated from the reference signal source is heterodyned and the frequency is decreased, so that frequency measurement is facilitated.
In the radar apparatus of the invention, the reference signal source includes a basic signal generation section for generating a basic signal, which is a source of the reference signal, and a frequency division section for dividing the frequency of the basic signal generated from the basic signal generation section according to one of frequency dividing ratios that can be switched to convert the basic signal into the reference signal.
According to the invention, the frequency dividing ratio of the frequency division section is changed for switching the frequency of the reference signal provided by dividing the frequency of the basic signal generated from the basic signal generation section, the frequency is lowered, and the frequency measurement accuracy can be enhanced.
In the radar apparatus of the invention, the reference signal source includes a plurality of reference signal generation sections and selects one of the reference signal generation sections to generate the reference signal.
According to the invention, a plurality of reference signal generation section are switched for generating the reference signal and the signal components provided by switching the reference signal generation section are compared, whereby it is made possible to determine the frequency to be measured, and the accuracy can be enhanced.
In the radar apparatus of the invention, the reference signal source comprises a signal arithmetic section for generating the reference signal by performing arithmetic processing according to a preset program.
According to the invention, the reference signal of the frequency fitted for frequency measurement can be generated for enhancing the measurement accuracy.
In the radar apparatus of the invention, the reference signal source supplies a clock signal for a signal arithmetic processing to the signal processing section.
According to the invention, generation of the reference signal and clock signal supply to the signal processing section can be conducted from the common reference signal source, so that space saving and cost reduction are made possible.
In the radar apparatus of the invention, a frequency of the reference signal is set so that a difference from the frequency of the reception signal from the target is within a beat signal band at a time of a radar operation.
According to the invention, the reference signal set within the beat signal band is used, so that the band of the frequency of the difference provided by heterodyning can be lowered for enhancing the measurement accuracy.
In the radar apparatus of the invention, the signal processing section performs at least a part of processing of the beat signal.
According to the invention, the configuration is shared between at least a part of beat signal processing and a part of detection processing of the frequency modulation characteristic, so that the whole configuration can be simplified and can be implemented at low costs.
In the radar apparatus of the invention, the signal processing section changes a predetermined time for holding a signal level of the modulation signal for detection in response to a distance to the target.
According to the invention, frequency modulation characteristic detection can be optimized in response to the distance to the target and the detection accuracy can be enhanced.
In the radar apparatus of the invention, the signal processing section changes a predetermined time for holding a signal level of the modulation signal for detection in response to a relative speed of the target.
According to the invention, the detection accuracy of the frequency modulation characteristic can be enhanced considering the relative speed with the target.
In the radar apparatus of the invention, the signal processing section detects the frequency of the differential signal between the high-frequency signal and the reception signal considering a Doppler shift based on a relative speed of the target.
According to the invention, if it is difficult to find out a relatively still object when the radar is installed in an automobile, etc., the frequency modulation characteristic can be detected with good accuracy considering the Doppler shift.